sharing myself...

Indian Railways provides an unique feature of current
reservation to the passengers to travel with confirmed tickets based on the
availability of the seats at the source station. Current reservation can be
done at the source station after the preparation of chart but before the
departure of train. After chart preparation the seats available for the current
reservation can be known at the counter next to the enquiry and not at the
regular advanced ticket reservation counter.

The information about current reservation can also be known
online by providing the name of the source station. The trains departing from
that station having the same station as the starting point in the next four
hours and the availability of tickets in those trains will be displayed. There
is no direct link on Indian Railways home page to access current reservation
enquiry page but can be got by searching for “current reservation Indian
Railways”.

Even the ticket issued for current reservation will be superscribed with “Journey cum Reservation ticket after charting”.

Sometimes current reservation may not be available due to
huge number of waiting list tickets. It is better to check online and make
alternate arrangements in case current reservation tickets are not available
instead of going to railway station. Also, it is useful for the people who plan
at the last moment and look for getting confirmed tickets.

Alternatively, IRCTC website (www.irctc.co.in) can also be used to book current reservation tickets after the initial chart preparation. As a benefit for passengers, IRCTC is offering the seats in current reservation at 10% discount on the reservation fare.

But what if the counter isn't open by the time of Train departure. I presume that counter will be open after 8 AM and train departure timing is 7 Am. Moreover, current booking isn't allowed before 10 AM.

The counter will be open always. But the ticket will be issued till half an hour before the departure of train. Now it has been made online. First chart will be generated three or four hours before train departure and second chart will be half an hour before train departure.